1. Field of the Invention
The present invention relates to an antenna for portable radio apparatuses, and in particular, relates to a helical antenna for portable radio apparatuses which receives signals in two separated communication bands or in a broad communication band.
2. Background Art
In conventional antennas for portable radio apparatuses, a technology for the miniaturization of the wire antenna was widely employed in which the emitting conductor was formed in a screw shape, producing a helical antenna. Furthermore, in concert with the reduction in size of portable radio apparatuses, it has become necessary to realize smaller helical antennas; however, as the size of the helical antennas is reduced, the band width becomes narrow, and it becomes difficult to cover the band width required of portable radio apparatuses.
In particular, in portable radio apparatuses having two or more separated communication bands, an antenna having an even wider band width is necessary in order to cover these bands, and this presents an obstacle to a reduction in size of the antenna. On the other hand, various methods have been considered for changing the antenna structure so as to provide two communication bands; however, these require approximately the same volume as providing two antenna elements, so that the need for size reduction is not met.
In order to make an antenna having a narrow communication band into an antenna capable of being employed for two separated communication bands, in Japanese Utility Model Application, First Publication No. Sho. 56-95107, a ferrite bar antenna is used, as shown in FIG. 8; by short circuiting a portion of coil wrapped around the bar and changing the resonance frequency of the antenna, two communication bands can be received. However, if the circuit structure shown in FIG. 8 is employed, the resonance frequency is determined by the number of turns of coil 23 and the capacity of capacitors 27 and 28 which are connected in parallel, so that in order to change the resonance frequency, a switch 26 for short circuiting a portion of the antenna and the ground, and a switch 29 for switching the capacitors, are necessary. Furthermore, by changing the resonance frequency, the position of the feeder terminal changes, so that it is also necessary to switch the feeder terminal, and switch 30 is required for this purpose, so that a total of 3 switches are necessary, and thus the communication band switching circuitry becomes complicated, requiring a large surface area for installation. The signals received in this manner are combined with a signal from a local oscillator 34 which is not depicted in the figure to create an intermediate frequency, and this is supplied to an intermediate frequency amplification circuit which is not depicted in the figure.
On the other hand, FIG. 9 shows an antenna disclosed in Japanese Utility Model Application, First Publication No. Hei. 4-10412. In this antenna, as shown in the figure, a conducting plate 56 is inserted within the helical antenna, shorting a portion of the helical antenna, and thereby changing the resonance frequency; however, when this method is employed, the resonance frequency can only be altered during the process of production, and such an antenna can not be used in portable radio apparatuses which conduct reception in such a manner as to switch the frequency of the antenna among two communication bands during use.
With the conventional methods such as those described above, the number of switches used to switch the resonance frequency of the antenna was large, and the circuitry became complex, so that the surface area required for installation of the portion used for switching the communication bands of the antenna was large, and the size of the antenna and the peripheral circuitry thereof increased, so that such methods were not applicable to portable radio apparatuses. Furthermore, when the antenna was made to have two resonance frequencies, this caused a problem in that volume of the antenna was excessively large.
The present invention was created in light of the above circumstances; it has as an object thereof to provide a helical antenna for portable radio apparatuses which is characterized in having a single communication band switching switch and having either two separated bands or a broad communication band.